The optical axis of the eye is characterized by the straight line between the centers of curvature of refractive surfaces, while the visual axis refers to the axis extending from the “fovea centralis,” through the nodal point of the eye, to the fixation object. If the various media are reduced by computation to a single medium having average refractive power and spherical curvature, a point in the eye may be indicated through which all beams pass uninterrupted. This point is referred to as the nodal point of the visual axes.
In all eyes, the visual axis generally deviates from the optical axis. This results, on the one hand, from aberrations of the eye, for example due to the fact that radii of curvature of the individual ocular media are not uniform, the lens of the eye is tilted, the retina is not situated in the focus of the lens of the eye, and many other factors. On the other hand, when the eye is aligned with an object, an attempt is made to image this object to the greatest extent possible in the fovea, which is the area of sharpest vision.
Although the alignment of the eye plays no role in many examinations in ophthalmology, at least the knowledge of its orientation with respect to the opthalmological device is absolutely necessary, not only for treatment, but also for measurement, of the eye.
The measurement of various parameters of an eye is necessary in particular prior to a surgical procedure for substituting an artificial lens of the eye (intraocular lens (IOL)) when clouding of the natural lens (cataract) is present. To ensure optimal vision after the procedure, these parameters must be determined with sufficiently high accuracy to subsequently allow a suitable replacement lens to be selected based on the determined measured values. The most important parameters to be determined include, among others, the axis length (distance from the cornea to the retina), the corneal curvature and refractive power, and the length of the anterior chamber (distance from the cornea to the lens of the eye).
Thus, for carrying out measurements on the eye it is advantageous for the optical axis of the opthalmological measuring system and the optical axis of the eye to be measured to be aligned with one another. In measurements according to the principle of short coherence interferometry, it may thus be ensured that the weak light components reflected from the boundary surfaces of the cornea and the lens reach the detector with adequate signal intensity and produce a measurable interference contrast.
The major technological advantage of OCT is the decoupling of the depth resolution from the transverse resolution. The depth resolution is determined only by the utilized bandwidth of the light source used. Common bandwidths are in the range of several nanometers to over one hundred nanometers, and when measuring radiation in the near infrared is used, 700-1350 nm. The depth resolutions thus achievable are in the range of 3-100 μm. In contrast to microscopy, the three-dimensional structure of the object to be examined may thus be detected, even when the numerical aperture, for example for small pupils in nondilated eyes, is greatly limited.
The purely reflective, and therefore contactless, measurement allows the generation of microscopic images of living tissue (in vivo). The wavelength of the measuring radiation to be used is determined by the desired application, taking into account the wavelength-dependent tissue absorption and back-scattering. If the ocular fundus, for example, is to be measured, in particular radiation in the range of 690-900 nm or 960-1100 nm is suitable, and for the anterior portion of the eye, for example radiation in the range of 1260-1360 nm is suitable.
Various approaches are known according to the prior art for interferometrically measuring the eye length and/or the anterior portion of the eye.
Thus, U.S. Pat. No. 7,380,939 B2 describes an approach for interferometric measurement of the anterior portion of the eye according to the so-called “dual beam” principle. This method requires careful adjustment of the measuring device and a targeted setting of the viewing direction of the patient. For this purpose, the eye is illuminated by a convergent beam bundle and aligned with the optical axis of the measuring system by generating directional stimuli and accommodation stimuli by use of a display which is reflected into the beam path. In clinical practice, adjusting these conditions is time-consuming, and with uncooperative patients is sometimes not possible at all.
An alternative approach is described in WO 2007/053971 A1, in which, instead of the reflected light resulting on the boundary surfaces in the eye, uses volume-scattered light which is back-scattered in a fairly large angular range. The volume-scattered light is usually detected in a diffraction-limited manner. This may preferably be carried out using optical single-mode fibers. However, the usable signal intensity is dependent on the scattering properties of the ocular media, and is generally much smaller than the directly reflected signal components. Patients who have already received an artificial lens cannot be measured in this manner.
A device is known from DE 198 57 001 A1 which may be used for contactless measurement of the eye length, corneal curvature, and depth of the anterior chamber. The axis length is determined interferometrically, the corneal curvature is determined by image processing based on reflected images from measuring marks projected onto the cornea at a certain angle, and the depth of the anterior chamber is determined from the evaluation of the back-scattering of slitted illumination of the lens of the eye. The described measurement of the depth of the anterior chamber does not function for pseudophakic eyes, since the implanted intraocular lenses (IOL) generally have no scattering effect.
For the measurement, the eye must be aligned in such a way that its optical axis coincides with the measuring axis of the device. To this end, collimated fixation light is directed onto the patient along a fixed (coaxial) axis, and is coupled via a mirror for the eye to be measured. An angle between the visual axis of the patient and the measuring axis of the test assembly is set using a scanning mirror.
Interferometric methods for measuring the eye length according to the “dual beam” principle are characterized by a high degree of suppression of axial motion artifacts. However, in order to record measuring variables in the anterior portion of the eye, such as the depth of the anterior chamber, lens thickness, etc. using the same method, there is the problem that light reflected from the particular boundary surface must be spatially superimposed on the reference reflection (usually the corneal reflection) in such a way that the interference of the partial beams is measurable. Due to the tilting of the lens of the eye which frequently occurs in humans, and thus the tilting of the visual axis with respect to the optical axis, the reflections of the various boundary surfaces generally are not situated on the same axis, and therefore cannot interfere with one another.
When the deviation of the optical axis from the measuring axis is in the range of 1° (for example, as the result of fixation problems or nystagmus), the reflections from the cornea and lens may no longer be superimposed, so that no interference measuring signal results when the “dual-beam” principle is used. The measurement is therefore very sensitive to tilting of the eye of the patient. In addition, the fixation light always appears to the patient at an infinite point, which may prove to be disadvantageous. The position of the optical axis is sought by tilting the scanning mirror in two mutually orthogonal directions until all measuring signals from the cornea and lens may be detected at the same time. This method is extremely time-consuming, and also does not provide the desired results in all patients. This method is laborious for use in everyday clinical practice.